GLACIATION OF THE YAKUTAT BAY REGION 209
Doubtless there are other factors which must be taken into account in a complete analysis of the phenomenon, and some of these are already evident. For instance, there must surely be steep slopes; but those are common enough and are present even in the valleys of glaciers which have no ablation moraine. Sufficiently steep slopes are almost invariably insured where dwindling glaciers occupy the beds of valleys overdeepened during a previous stage of greater expansion. Without steep slopes, such as glacial erosion provides, extensive ablation moraines are not possible; but that their presence alone is not a sufficient cause is abundantly proved by the absence of such moraines in valleys whose walls are as steep as those of moraine-covered glaciers. Another factor that is certainly important is friability of rock. The best ablation moraines of the Yaku-tat Bay region are in the friable Yakutat shales, sandstones, and conglomerates; but they are also present on glaciers, like the Variegated, which descend through valleys in crystalline rocks. It is probable, however, that extensive ablation moraines, such as those of Yakutat Bay, could not develop on glaciers whose entire course was through valleys enclosed in massive gneiss or granite. We are inclined to believe that the absence of ablation moraine on many small glaciers, enclosed between steeply-rising valley walls, like some of the Alpine glaciers, is due mainly to the stability of the valley wall rock.
A third possible factor, naturally suggested in this region, is that of earthquake ava-lanching. Valley walls are greatly steepened by glacial erosion, especially in the weaker friable rocks, and by shrinking of the glaciers these too-steep walls are exposed to subsequent weathering. One finds abundant evidence on every hand that weathering is working rapidly to reduce such slopes, for avalanching is commonly observed, and the rock is crossed by rifts and gashes, where weathering is preparing masses to slide down the steepened slopes. Sometimes these gashes in the upper part of the steepened valley slope are so wide that one cannot cross them, and scores of yards in length. When an earthquake comes there are many such masses ready to fall under the impulse of the associated shaking; and we have abundant evidence that great numbers of such masses did fall during the earthquakes of 1899. Such masses may often be of sufficient size to spread completely across a valley glacier. If a glacier bearing such a load is caused to advance by earthquake avalanching, we have a means of quickly moving down the valley the rock masses which the avalanching supplied to the glaciers; and if the piedmont bulbs themselves are the product of a great advance under the earthquake impulse, as is possible, the broad sheets of ablation moraine which cover them may in part be due to this cause. Earthquake shaking would have the double effect of quickly rolling to the glacier surface a larger amount of material than mere weathering would supply in the same time, and of contributing it in such great individual avalanches that it could spread farther out on the glaciers than would be common in those avalanches caused by weathering alone. The aid of earthquakes is not essential, but it simplifies the process and is perhaps an important factor. It would also explain the fact which we observed in Atrevida Glacier, that the area of ablation moraine desert extended farther up the glacier in 1909 than it did in 1905.
Interior Flats. In the description of Variegated Glacier it is shown that there is a • level area, crescentic in form, and bordered by moraine-covered ice which rises steeply 100 to 150 feet above the plain. This level area is evidently a part of the glacier which for some reason had a thinner moraine cover and, therefore, wasted so much faster than the neighboring parts of the ice that it had, by 1905, become the seat of alluvial